From the viewpoint of environmental protection of city and roadside air and reduction of environmental burden on a global scale, it has been required to clean exhaust gas from automobiles and reduce the discharge of carbon dioxide. With regard to gasoline-powered automobiles, in order to reduce the discharge of carbon dioxide, it is required to improve fuel economy. Thus, thereafter, it is expected that high mileage automobiles having a direct-injection engine or a lean burn engine may increase. For these automobiles, a nitrogen oxide-reducing catalyst working under oxygen-excessive conditions (lean conditions) is necessary, which is different from a ternary catalyst which is a conventional nitrogen oxide-reducing catalyst. However, since the nitrogen oxide-reducing catalyst working under oxygen-excessive conditions (lean conditions) is poisoned by sulfur content and cleaning performance lowers, it is still required to achieve further decrease of the sulfur content in gasoline.
Currently, commercially available gasoline is prepared by mixing various base materials so as to satisfy the standard of an octane number. Among the materials, a catalytic cracking gasoline obtained by cracking, in a catalytically cracking apparatus, vacuum gas oil and atmospheric residue whose uses are limited as they are base materials having a high octane number and a mixing ratio thereof in the commercially available gasoline is as high as 40 to 70%. However, since starting materials of the catalytic cracking gasoline are vacuum gas oil and atmospheric residue containing a large amount of sulfur, the sulfur content of the resulting catalytic cracking gasoline is also higher than other base materials for gasoline. Therefore, in order to reduce the sulfur content of the commercially available gasoline produced by mixing various base materials, it is indispensable to desulfurize the catalytic cracking gasoline which is a main base material.
For reducing the sulfur content of the catalytic cracking gasoline, it is a common practice to use the vacuum gas oil and atmospheric residue after hydrodesulfurization as starting oils for catalytic cracking. However, an apparatus for the hydrodesulfurization of these heavy oils requires a high temperature and a high pressure and hence there exist many technical and economical problems in ultra-deep desulfurization of the staring oils.
Sulfur compounds contained in the catalytic cracking gasoline can be hydrodesulfurized by an apparatus requiring a relatively low temperature and a low pressure. Therefore, it is advantageous, if possible to directly hydrodesulfurize the catalytic cracking gasoline, since the process may be not only economically inexpensive but also can respond the case where the sulfur content of the starting oils for catalytic cracking is high.
With regard to the hydrotreating catalyst for the catalytic cracking gasoline, there have been conventionally widely used sulfide catalysts obtained by preparing an impregnating solution comprising a compound of molybdenum, a compound of a Group 8 metal of the periodic table, such as cobalt or nickel, and/or a phosphorus compound and supporting the solution in an oxide form on a porous inorganic oxide support such as alumina, silica, or zeolite, followed by activation through presulfuration. It is considered that the active sites for desulfurization in these sulfide catalysts are sulfur-coordinating unsaturated sites exhibited on a Group 8 metal-Group 6 metal-S structure (e.g., Co—Mo—S structure) formed at the edge site of a sulfide layer of molybdenum or tungsten which is a Group 6 metal. In order to improve the desulfurization activity, an attempt has been made to highly disperse the Group 6 metal sulfide. However, there arises a problem that when e catalytic cracking gasoline is hydrodesulfurized by a conventional hydrodesulfurization catalyst, olefins contained as high octane number components in the catalytic cracking gasoline are hydrogenated and thus the octane number decreases.
To overcome this problem, the present inventors previously proposed, as a hydrotreating catalyst having a low activity of hydrogenating olefins catalytic cracking gasoline and an excellent desulfurization performance, a hydrotreating catalyst of catalytic cracking gasoline, which comprises a Group 8 metal of the periodic table, molybdenum (Mo), phosphorus and sulfur, wherein the average coordination number [N(Mo)] of the molybdenum atoms around the molybdenum atom is from 1.5 to 2.5 and the average coordination number [N(S)] of the sulfur atoms around the molybdenum atom is from 3.5 to 5.0 when the MoS2 structure in the catalyst is measured in accordance with the extended X-ray absorption fine structure (EXAFS) analysis (Patent Document 1).
Patent Document 1: JP-A-2005-329320